A Tetrahydrofolate-Dependent Methyltransferase Catalyzing the Demethylation of Dicamba in Sphingomonas sp. Strain Ndbn-20

Sphingomonas sp. strain Ndbn-20 degrades and utilizes the herbicide dicamba as its sole carbon and energy source. In the present study, a tetrahydrofolate (THF)-dependent dicamba methyltransferase gene, dmt, was cloned from the strain, and three other genes, metF, dhc, and purU, which are involved in THF metabolism, were found to be located downstream of dmt A transcriptional study revealed that the four genes constituted one transcriptional unit that was constitutively transcribed. Lysates of cells grown with glucose or dicamba exhibited almost the same activities, which further suggested that the dmt gene is constitutively expressed in the strain. Dmt shared 46% and 45% identities with the methyltransferases DesA and LigM from Sphingomonas paucimobilis SYK-6, respectively. The purified Dmt catalyzed the transfer of methyl from dicamba to THF to form the herbicidally inactive metabolite 3,6-dichlorosalicylic acid (DCSA) and 5-methyl-THF. The activity of Dmt was inhibited by 5-methyl-THF but not by DCSA. The introduction of a codon-optimized dmt gene into Arabidopsis thaliana enhanced resistance against dicamba. In conclusion, this study identified a THF-dependent dicamba methyltransferase, Dmt, with potential applications for the genetic engineering of dicamba-resistant crops.

Dicamba is a very important herbicide that is widely used to control more than 200 types of broadleaf weeds and is a suitable target herbicide for the engineering of herbicide-resistant transgenic crops. A study of the mechanism of dicamba metabolism by soil microorganisms will benefit studies of its dissipation, transformation, and migration in the environment. This study identified a THF-dependent methyltransferase, Dmt, capable of catalyzing dicamba demethylation in Sphingomonas sp. Ndbn-20, and a preliminary study of its enzymatic characteristics was performed. Introduction of a codon-optimized dmt gene into Arabidopsis thaliana enhanced resistance against dicamba, suggesting that the dmt gene has potential applications for the genetic engineering of herbicide-resistant crops.